Human monocytic ehrlichiosis (HME) caused by Ehrlichia chaffeensis was first reported in 1987. The clinical symptoms of HME include fever, headache, malaise, myalgia, rash, lymphadenopathy, and nausea (Rikihisa, 1999). Illness due to ehrlichiosis can be so mild that no medical care is sought, or the illness can be severe and sometimes fatal, particularly in the immune compromised and elderly. Symptoms are generally non-specific, and other diagnoses may be considered. Because the laboratory tests that detect ehrlichiosis are often not positive in the first week of illness, physicians base early patient treatment decisions on the signs and symptoms, as well as the patient's history of tick exposure. The physician also looks at specific blood tests to help determine the likelihood of ehrlichiosis. Clues such as a low platelet count (thrombocytopenia), abnormal white blood cell counts (decreased), or elevated liver enzyme levels are often helpful, yet non-specific predictors.
As yet there are no vaccines to protect against ehrlichiosis. Hence there is a need to develop a potential vaccine that can protect against Ehrlichia infection.